Many states have stringent environmental laws that include a requirement that air exiting from an exhaust stack or air exhaust duct is discharged into the air at a predetermined height (or distance from intake ducts) and a preset velocity to ensure proper diffusion in the air of any chemicals that may be contained in the discharged air. For example, some states calculate the required discharge velocity based on the type of chemicals that are used in the facility. These discharge velocities from the stacks or ducts are generally in the range of 3000 to 3600 feet per minute.
In theory, this high discharge velocity high in the air dilutes the chemicals before they reach an area where someone might come into contact with them. As a result, the chemicals will not re-enter the facility in any measurable quantity. If the chemicals did re-enter, they might cause a health hazard to the occupants. Most of the regulations are based on a worse-case analysis, and often the discharge velocity is high enough that even a much greater percentage of chemicals than is allowed could be contained in the output and still be safe.
One problem facing many facilities that are covered by these regulations is the large constant volume of air which must be discharged from the facility to maintain velocity. For example, some research-based companies may have many thousands of individual inventors and researchers, each having a chemical fume hood which exhausts a large amount of air. The combination of many chemical fume hoods working at once creates a large volume of air that must be discharged through the stack or duct to maintain a high discharge velocity and ensure compliance with environmental regulations.
However, the amount of air which is required to be discharged from the chemical fume hoods and other sections of the facility may vary considerably during the day, and thus, the volume of air discharged from the facility could, if allowed, vary considerably over a daily period--thus, the popularity of variable volume exhaust systems which allow fume hoods t o use only the air that is required for research safety. For instance, at lunch time, the volume of discharged air could be reduced dramatically because many chemical fume hoods would be closed while workers take their lunch breaks. However, when the regulations require the velocity of discharged air to remain the same throughout the day, ambient air must be pumped into the stack to ensure constant discharge velocity. During those periods when many fume hoods and other parts of the facility are not discharging heavily, a large amount of ambient air must be pumped into the stack, increasing considerably the energy costs to run the facility. Even though the percentage of contaminants may be very small, the high discharge velocity is often maintained to ensure compliance with the environmental regulations and to minimize re-entrainment of exhaust into the building.
It would be advantageous if the energy costs associated with the maintenance of a high velocity discharge could be reduced by reducing the amount of ambient air that must be diverted into the stacks to maintain the required velocity through the stack outlet. Some straight baffle damper designs have been tried, but these designs have not been successful because the high velocity of air creates eddy currents behind the straight damper blades causing interior plume development (low level dispersion) and vibration. This vibration decreases the operating life of the stack. Also, the vibration of straight baffles causes a noise problem that could violate environmental regulations.
Examples of prior art in damper valves and devices which regulate flow through an exhaust shaft or duct include U.S. Pat. No. 1,358,854 to Kendall; U.S. Pat. No. 4,207,864 to Fischer et al.; U.S. Pat. No.4,269,166 to Worley et al.; and U.S. Pat. No. 4,337,892 to Diermayer et al. None of these devices maintain a constant discharge velocity through a stack or duct when the total amount of discharged air decreases such as when chemical fume hoods shut down.